1. Field of the Invention
This invention relates to an optical amplifier for amplifying a signal light and a continuous light having a different wave length. More particularly, the present invention relates to an optical amplifier which functions to couple a signal light and a continuous light and amplifying the coupled light so as to reduce the effect of ASE (Amplified Spontaneous Emission) generated in the optical fiber amplifier, to expand dynamic range of the output light and to alleviate the deterioration of the extinction ratio when the signal light is pulse-modulated light.
2. Prior Art
The conventional optical amplifier will first be briefly explained by making reference to FIG. 7. In this figure, 1 is an optical source 1 which outputs a signal light 11 having a wave length .pi. a, 4 is an optical fiber amplifier and 5 is an optical filter. The optical fiber amplifier 4 amplifies the signal light 11 from the optical source 1. The output of the optical fiber amplifier 4 includes ASE generated in the signal light 11 and the optical fiber amplifier 4. Detected light involves beat noise between them as well as that among the spectral components of the ASE. The optical filter 5 extracts the light component having a wavelength of .pi. a from the output of the optical fiber amplifier 4, whereby the beat noise resulting from the spectral components of the ASE is reduced.
Next, the input/output characteristics of the system in FIG. 7 will be explained in reference to FIG. 8. In this figure, the abscissa is the input optical power and the ordinate is the output optical power. When the input optical power is small the output optical power increases with the increase in the input optical power. When the input optical power becomes sufficiently large the output light power reaches a saturation as depicted by a power curve 16. The level 17 of the curve 16 is the minimum light receipt optical power which is restricted by the beat noise between the signal light 11 and the beat noise between the spectral components of the ASE, and the level 18 is the maximum receipt optical power right prior to the saturation of the output signal 11 of the optical fiber amplifier 4. Incidentally, beat noise is described in FIG. 9 of Masataka Nakazawa "Optical Fiber Amplification with Er-Doped Optical Fiber and Its Application "Applied Physics of Japan",Vol. 59, No.9 (1990).
Problem to be solved by the present invention
With the construction in FIG. 7, even if it is desired to use the signal light 11 in a wide dynamic range, the power of the ASE is large when the power of the signal light 11 is small and, accordingly, it is not possible to expand the dynamic range, that is, the range between the level 17 and the level 18 shown in FIG. 8. In addition, if the signal light 11 has been pulse-modulated, the extinction ratio is deteriorated by the ASE in the pass band of the optical filter 5.
Accordingly, a principal object of the invention is to reduce the effect of ASE generated in the optical fiber amplifier 4 so as to expand the dynamic range of the output light, and, in the case of the signal light being pulse-modulated, deterioration of the extinction ratio is suppressed.
Another object of the present invention is to provide an optical amplifier which amplifies the signal light 11 having a wavelength .pi. a simultaneously with another continuous light having a wavelength .pi. b in the optical fiber amplifier 4 and only the light having the wavelength .pi. a is selected by the optical filter 5, which achieves the above-mentioned object of the invention.
Means to solve the problem.
To achieve the objects, the present invention provides an optical fiber amplifier comprising an optical source for generating a signal light having a wavelength .pi. a, an optical source for generating a continuous light having a wavelength .pi. b, an optical coupler for coupling the signal light with the continuous light, an optical fiber amplifier for amplifying the output from the optical coupler, and a optical filter for extracting only the light having the wavelength .pi. a.